I've heard what I thought was some pretty decent guitar playing in modern Yamaha styles. To be honest, I am prepared to admit that they are, short of actual audio loops, about the best that the state of the art can get them without using VSTi's.

Which Yamaha are you using?

The problem with rock guitar MIDI's is that it is ALL about the amp simulator. It's the way that chords make the amp break up, single lines sing out, power chords (no thirds) get crunchy. And about the Mega Voice information, rakes, scrapes, pick noise, etc., etc.. And lastly, and most difficultly, it is about the way that chords get played on a guitar. MIDI arrangers tend to simply transpose guitar parts, a part that you really liked from a MIDI file will NOT work well when transposed to all keys, let alone all chords. Guitars are very non-linear things. A G chord is NOTHING like a C chord. Different shape, different chord. Until you get into barré chords, no two chords get played the same way, and most guitar stuff (or at least, a whole lot of it!) is played down at the nut end of the guitar, where everything is different.

Yamaha's T3 and S910 have new NTT's to help with making their guitar parts more natural when transposed. Do you have one of these, yet?

I'm afraid what you want to do simply is beyond much of today's technology, at least if you have already auditioned Yamaha's current styles and find them lacking. Only other thing I can suggest is to try Korg's PA800/PA2Xpro, which has a dedicated Guitar Mode in it that makes a pretty good job of picking the correct voicing for different key, same type chords...

Hate to be the bearer of bad news, but I think you are asking a bit much from today's arrangers, even as good as I think they are...